1. Field
The present invention relates generally to data communication, and more specifically to techniques for generating a reference signal, comprised of interpolated pilot symbols, for a gated pilot and with compensation for phase changes induced in a received signal.
2. Background
In a typical wireless communication system (e.g., a CDMA system), data is processed, coded, modulated, filtered, amplified, and transmitted from a transmitter unit (e.g., a base station) to one or more receiver units (e.g., terminals). The transmitted signal typically experiences path loss, multipath, and other phenomena prior to reaching the receiver units. At each receiver unit, the transmitted signal is received, conditioned, demodulated, and decoded to recover the transmitted data. The signal conditioning typically includes amplification, filtering, frequency downconversion, and so on.
Due to path loss, multipaths (which may add constructively or destructively), and other phenomena, the received signal strength may vary over a wide range of values. For example, for a CDMA system, the received signal strength can vary over 85 dB. To provide a conditioned signal having the proper amplitude for digitization, multiple stages of low noise amplifier (LNA) and/or variable gain amplifier (VGA) are usually employed in the received signal path from the antenna to the analog-to-digital converters (ADCs). The received signal path may also include one or more sections in which the received signal can be routed through one of several possible signal paths (e.g., a gain path and a bypass path for an LNA stage).
The characteristics of the received signal can change as circuit elements in the received signal path are adjusted and/or as different signal paths are selected. Each circuit element in the received signal path is associated with a particular phase, and each setting of the circuit element may also be associated with a different phase. The received signal can have phase discontinuities if the circuit elements are adjusted in a step-wise fashion (e.g., for a VGA with gain steps) or if the received signal is switched through different signal paths (e.g., the gain and bypassed paths for an LNA stage).
In many wireless communication systems, a pilot is transmitted from each transmitter unit to assist the receiver units perform a number of functions. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., covered with a known channelization code and spread with a known pseudo-random noise (PN) sequence or scrambling code). The pilot may be used at the receiver units for synchronization with the timing and frequency of the transmitter unit, estimation of the quality of the communication channel, coherent demodulation of a data transmission, and possibly other functions.
Some CDMA systems (e.g., IS-856 and W-CDMA systems) employ a gated pilot transmission scheme. For these systems, the pilot is transmitted in bursts at regular time intervals. At each receiver unit, the pilot bursts are processed to obtain a recovered pilot symbol for each pilot burst, with the recovered pilot symbol being an estimate of the amplitude and phase of the pilot (and thus the communication channel) at a particular time instance. A (continuous) reference signal may then be generated by interpolating the recovered pilot symbols, and this reference signal may be used for data demodulation and other purposes. If phase discontinuities are induced in the received signal due to circuit changes in the received signal path, as described above, then these phase discontinuities need to be accounted for in the reference signal so that data demodulation is not degraded.
There is therefore a need in the art for techniques to generate a reference signal for a gated pilot in a manner to account for phase changes induced in the received signal due to adjustments and/or switching of circuit elements in the received signal path.